1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for forming contact holes for conductive strips passing through an insulating film, such as an interlayer insulating film.
2. Description of Related Art
In semiconductor devices, including semiconductor elements, such as MOS transistors, when contact holes, which run through an insulating film, where the MOS transistors are buried, and which open to active regions on the semiconductor substrate, are formed between gate electrodes of a pair of MOS transistors on a semiconductor substrate, for example, they are generally formed by a self-aligned contact process.
According to the self-aligned contact process, a protective film of silicon nitride, for example, is formed to cover gate electrodes for MOS transistors on a semiconductor substrate, for example. Moreover, an insulating film of silicon oxide is formed to bury the gate electrodes along with the protective film in it. On this insulating film, wiring for a multi-layered structure is formed when necessary, and prior to this wiring work, to form contact holes that open to the semiconductor substrate between the gate electrodes, those regions of the insulating film which are located between the gate electrodes are subjected to an etching process using an etching mask and an etching gas to expose the above-mentioned regions between the gate electrodes.
Because the etching gas shows a notably low etching rate to the protective film than to the insulating film, the protective film serves to protect the gate electrodes from the etching gas during etching. When the side-wall portions of the protective film which cover the side walls of the gate electrodes are exposed in the course of the etching process, the side-wall portions function as etching stoppers, and contact holes can be formed between the gate electrodes with high accuracy regardless of placement accuracy of the etching mask, so that specified conductive property can be obtained by a conductive material used to fill the contact holes.
Meanwhile, in conventional semiconductor devices such as have been mentioned above, it is often required to provide two contact holes: a contact hole that opens to the surface of the semiconductor substrate and another contact hole that opens to a conductor, such as a gate electrode under the protective film.
In the former, the contact hole can be formed by a single etching process in a specified position of the insulating film. In the latter, however, after the insulating film has been etched, the exposed protective film needs to be etched. Because the protective film has a different etching-resistant characteristic from that of the insulating film, their etching conditions differ to a large extent in the forming process of the two contact holes.
For this reason, in the conventional manufacturing method, it is necessary to separately carry out an etching process using a mask for the contact hole that opens to the surface of the semiconductor substrate, and an etching process using a mask for the contact hole that opens to the conductive line under the protective film. Therefore, it is necessary to set a relative position of the two masks for the different contact holes with accuracy.
In this respect, there has been requirement for a method for forming two contact holes with high accuracy and with improved facility.
To solve the above problem, the present invention adopts the following structure.
In manufacturing a semiconductor device having a semiconductor substrate, a conductor formed on the semiconductor substrate and covered with a protective film, and an insulating film deposited above the semiconductor substrate to cover the conductor, a method for forming a first contact hole running through the insulating film and opening to the surface of the semiconductor substrate or to an electric connection member buried in the insulating film, and a second contact hole laterally positioned relative to the first contact hole running through both the insulating film and the protective film and opening to said conductor under the protective film, the contact-hole forming method comprises the steps of:
forming an etched-away opening running from the upper surface of the insulating film to expose the top portion of the conductor corresponding to the opening by using an etching medium exhibiting a higher etching rate to the protective film than to the insulating film to partially remove by etching the protective film on the conductor thereby defining the location of the second contat hole;
refilling the etched-away opening with a material of the same characteristic as the insulating film; and
etching the layer of the insulating film that has been refilled with a single etching medium to thereby simultaneously form the first contact hole opening to the semiconductor substrate or the electric connection member buried in the insulating film, and the second contact hole opening to the conductor.
According to the above method of the present invention, the protective film above the conductor, where the second contact hole is to be formed, is removed in advance, when the etched-away opening is formed to expose the top portion of the corresponding conductor from the surface of the insulating film, and the etched-away opening is refilled. Under this condition, it is possible to form the second contact hole substantially under the same condition as in etching of the first contact hole.
Moreover, in the formation of the etched-away opening, because an etching medium is used which exhibits a greater etching rate to the protective film than to the insulating film, the etched-away opening does not have its circumferential wall become like a beer barrel or taper off nor does it incur an etching stoppage at the interface between the insulating film and the protective film caused by a change of the kind of etching medium, so that the protective film can be etched so as to have specified parts appropriately removed.
Therefore, according to the above-mentioned method of the present invention, a final etching process to form the first and second contact holes after refilling of the etched-away opening can be carried out substantially under the same condition, and therefore both contact holes can be formed collectively by an etching process using a single etching mask having arranged therein two opening patterns for the first and second contact holes.
The above-mentioned conductors include gate electrodes or signal lines of field-effect semiconductor devices formed on a semiconductor substrate and various kinds of conductive parts formed on the semiconductor substrate.
The etching rate of the etching medium with respect to the protective film can be made 1.3xcx9c2.0 times that of the insulating film.
When the insulating film is silicon oxide and the protective film is a silicon nitride film used as an etching stopper in a self-aligned contact process, a mixed gas of CHF3 and CO as the etching medium.